Radiative torques alignment in the presence of pinwheel torques

We study the alignment of grains subject to both radiative torques and pinwheel torques while accounting for thermal flipping of grains. By pinwheel torques we refer to all systematic torques that are fixed in grain body axes, including the radiative torques arising from scattering and absorption of isotropic radiation. We discuss new types of pinwheel torques, which are systematic torques arising from infrared emission and torques arising from the interaction of grains with ions and electrons in hot plasma. We show that both types of torques are long-lived, i.e. may exist longer than gaseous damping time. We compare these torques with the torques introduced by E. Purcell, namely, torques due to H$_2$ formation, the variation of accommodation coefficient for gaseous collisions and photoelectric emission. Furthermore, we revise the Lazarian & Draine model for grain thermal flipping. We calculate mean flipping timescale induced by Barnett and nuclear relaxation for both paramagnetic and superparamagnetic grains, in the presence of stochastic torques associated with pinwheel torques, e.g. the stochastic torques arising from H$_2$ formation, and gas bombardment. We show that the combined effect of internal relaxation and stochastic torques can result in fast flipping for sufficiently small grains and, because of this, they get thermally trapped, i.e. rotate thermally in spite of the presence of pinwheel torques. For sufficiently large grains, we show that the pinwheel torques can increase the degree of grain alignment achievable with the radiative torques by increasing the magnitude of the angular momentum of low attractor points and/or by driving grains to new high attractor points.Comment: 23 pages and 15 figures emulated ApJ style. Thermal flipping and trapping revised; paper accepted to Ap

Thermal Flipping and Thermal Trapping -- New Elements in Dust Grain Dynamics

Since the classical work by Purcell (1979) it has been generally accepted that most interstellar grains rotate suprathermally. Suprathermally rotating grains would be nearly perfectly aligned with the magnetic field by paramagnetic dissipation if not for ``crossovers'', intervals of low angular velocity resulting from reversals of the torques responsible for suprathermal rotation; during crossovers grains are susceptible to disalignment by random impulses. Lazarian and Draine (1997) identified thermal fluctuations within grain material as an important component of crossover dynamics. For grains of size less than 0.1 micron, these fluctuations ensure good correlation of angular momentum before and after crossover resulting in good alignment, in accord with observations of starlight polarization. In the present paper we discuss two new processes which are important for the dynamics of grains with a<0.1 micron. The first -- ``thermal flipping'' -- offers a way for small grains to bypass the period of greatly reduced angular momentum which would otherwise take place during a crossover, thereby enhancing the alignment of small grains. The second effect -- ``thermal trapping'' -- arises when thermal flipping becomes rapid enough to prevent the systematic torques from driving the grain to suprathermal rotation. This effect acts to reduce the alignment of small grains. The observed variation of grain alignment with grain size would then result from a combination of the thermal flipping process -- which suppresses suprathermal rotation of small grains -- and due to molecular hydrogen formation and starlight -- which drive large grains to suprathermal rotation rates.Comment: 16 pages, 2 figures, submitted ApJ

Radiative torque alignment: Essential Physical Processes

We study the physical processes that affect the alignment of grains subject to radiative torques (RATs). To describe the action of RATs, we use the analytical model (AMO) of RATs introduced in Paper I. We focus our discussion on the alignment by anisotropic radiation flux with respect to magnetic field, which defines the axis of grain Larmor precession. Such an alignment does not invoke paramagnetic dissipation (i.e. Davis-Greenstein mechanism), but, nevertheless, grains tend to be aligned with long axes perpendicular to the magnetic field. When we account for thermal fluctuations within grain material, we show that for grains, which are characterized by a triaxial ellipsoid of inertia, the zero-$J$ attractor point obtained in our earlier study develops into a low-$J$ attractor point. We study effects of stochastic gaseous bombardment and show that gaseous bombardment can drive grains from low-$J$ to high-$J$ attractor points in cases when the high-$J$ attractor points are present. As the alignment of grain axes with respect to angular momentum is higher for higher values of $J$, counter-intuitively, gaseous bombardment can increase the degree of grain alignment in respect to the magnetic field. We also study the effects of torques induced by H$_2$ formation and show that they can change the value of angular momentum at high-$J$ attractor point, but marginally affect the value of angular momentum at low-$J$ attractor points. We compare the AMO results with those obtained using the direct numerical calculations of RATs acting upon irregular grains and validate the use of the AMO for realistic situations of RAT alignment.Comment: 31 pages. MNRAS 2007, in press, typos are corrected

Tsallis statistics as a tool for studying interstellar turbulence

We used magnetohydrodynamic (MHD) simulations of interstellar turbulence to study the probability distribution functions (PDFs) of increments of density, velocity, and magnetic field. We found that the PDFs are well described by a Tsallis distribution, following the same general trends found in solar wind and Electron MHD studies. We found that the PDFs of density are very different in subsonic and supersonic turbulence. In order to extend this work to ISM observations we studied maps of column density obtained from 3D MHD simulations. From the column density maps we found the parameters that fit to Tsallis distributions and demonstrated that these parameters vary with the sonic and Alfv\'en Mach numbers of turbulence. This opens avenues for using Tsallis distributions to study the dynamical and perhaps magnetic states of interstellar gas.Comment: 8 pages, 5 figures, 1 table. Accepted for publication in the Astrophysical Journa

Electric dipole moments and disalignment of interstellar dust grains

The degree to which interstellar grains align with respect to the interstellar magnetic field depends on disaligning as well as aligning mechanisms. For decades, it was assumed that disalignment was due primarily to the random angular impulses a grain receives when colliding with gas-phase atoms. Recently, a new disalignment mechanism has been considered, which may be very potent for a grain that has a time-varying electric dipole moment and drifts across the magnetic field. We provide quantitative estimates of the disalignment times for silicate grains with size > approximately 0.1 micron. These appear to be shorter than the time-scale for alignment by radiative torques, unless the grains contain superparamagnetic inclusions.Comment: 12 pages, 9 figures, submitted to MNRA

Interstellar Sonic and Alfv\'enic Mach Numbers and the Tsallis Distribution

In an effort to characterize the Mach numbers of ISM magnetohydrodynamic (MHD) turbulence, we study the probability distribution functions (PDFs) of patial increments of density, velocity, and magnetic field for fourteen ideal isothermal MHD simulations at resolution 512^3. In particular, we fit the PDFs using the Tsallis function and study the dependency of fit parameters on the compressibility and magnetization of the gas. We find that the Tsallis function fits PDFs of MHD turbulence well, with fit parameters showing sensitivities to the sonic and Alfven Mach numbers. For 3D density, column density, and position-position-velocity (PPV) data we find that the amplitude and width of the PDFs shows a dependency on the sonic Mach number. We also find the width of the PDF is sensitive to global Alfvenic Mach number especially in cases where the sonic number is high. These dependencies are also found for mock observational cases, where cloud-like boundary conditions, smoothing, and noise are introduced. The ability of Tsallis statistics to characterize sonic and Alfvenic Mach numbers of simulated ISM turbulence point to it being a useful tool in the analysis of the observed ISM, especially when used simultaneously with other statistical techniques.Comment: 20 pages, 16 figures, ApJ submitte

Studying Turbulence using Doppler-broadened lines: Velocity Coordinate Spectrum

We discuss a new technique for studying astrophysical turbulence that utilizes the statistics of Doppler-broadened spectral lines. The technique relates the power Velocity Coordinate Spectrum (VCS), i.e. the spectrum of fluctuations measured along the velocity axis in Position-Position-Velocity (PPV) data cubes available from observations, to the underlying power spectra of the velocity/density fluctuations. Unlike the standard spatial spectra, that are function of angular wavenumber, the VCS is a function of the velocity wave number k_v ~ 1/v. We show that absorption affects the VCS to a higher degree for small k_v and obtain the criteria for disregarding the absorption effects for turbulence studies at large k_v. We consider the retrieval of turbulence spectra from observations for high and low spatial resolution observations and find that the VCS allows one to study turbulence even when the emitting turbulent volume is not spatially resolved. This opens interesting prospects for using the technique for extragalactic research. We show that, while thermal broadening interferes with the turbulence studies using the VCS, it is possible to separate thermal and non-thermal contributions. This allows a new way of determining the temperature of the interstellar gas using emission and absorption spectral lines.Comment: 27 page, 3 figures, content extended and presentation reorganized to correspond to the version accepted to Ap

Velocity Field of Compressible MHD Turbulence: Wavelet Decomposition and Mode Scalings

We study compressible MHD turbulence, which holds key to many astrophysical processes, including star formation and cosmic ray propagation. To account for the variations of the magnetic field in the strongly turbulent fluid we use wavelet decomposition of the turbulent velocity field into Alfven, slow and fast modes, which presents an extension of the Cho & Lazarian (2003) decomposition approach based on Fourier transforms. The wavelets allow to follow the variations of the local direction of magnetic field and therefore improve the quality of the decomposition compared to the Fourier transforms which are done in the mean field reference frame. For each resulting component we calculate spectra and two-point statistics such as longitudinal and transverse structure functions, as well as, higher order intermittency statistics. In addition, we perform the Helmholtz-Hodge decomposition of the velocity field into the incompressible and compressible parts and analyze these components. We find that the turbulence intermittency is different for different components and we show that the intermittency statistics depend on whether the phenomenon was studied in the global reference frame related to the mean magnetic field or it was studied in the frame defined by the local magnetic field. The dependencies of the measures we obtained are different for different components of velocity, for instance, we show that while the Alfven mode intermittency changes marginally with the Mach number the intermittency of the fast mode is substantially affected by the change.Comment: 16 pages, 9 figures, 2 table

Galactic foregrounds: Spatial fluctuations and a procedure of removal

Present day cosmic microwave background (CMB) studies require more accurate removal of Galactic foreground emission. In this paper, we consider a way of filtering out the diffuse Galactic fluctuations on the basis of their statistical properties, namely, the power-law spectra of fluctuations. We focus on the statistical properties of two major Galactic foregrounds that arise from magnetized turbulence, namely, diffuse synchrotron emission and thermal emission from dust and describe how their power laws change with the Galactic latitude. We attribute this change to the change of the geometry of the emission region and claim that the universality of the turbulence spectrum provides a new way of removing Galactic foregrounds. We discuss and demonstrate how we can make use of our findings to remove Galactic foregrounds using a template of spatial fluctuations. In particular, we consider examples of spatial filtering of a foreground at small scales, when the separation into CMB signal and foregrounds is done at larger scales. We demonstrate that the new technique of spatial filtering of foregrounds may be promising for recovering the CMB signal in a situation when foregrounds are known at a scale different from the one under study. It can also improve filtering by combining measurements obtained at different scales.Comment: 24 pages, 12 figures, accepted to Ap

Magnetic reconnection as the cause of cosmic ray excess from the heliospheric tail

The observation of a broad excess of sub-TeV cosmic rays compatible with the direction of the heliospheric tail (Nagashima et al. 1998) and the discovery of two significant localized excess regions of multi-TeV cosmic rays by the MILAGRO collaboration (Abdo et al. 2008), also from the same region of the sky, have raised questions on their origin. In particular, the coincidence of the most significant localized region with the direction of the heliospheric tail and the small angular scale of the observed anisotropy (~ 10deg) is suggestive a local origin and of a possible connection to the low energy broad excess. Cosmic ray acceleration from magnetic reconnection in the magnetotail is proposed as a possible source of the energetic particles.Comment: 10 pages, 5 figures, accepted for publication in Ap